Recent practice has shown the efficacy, both for diagnostic purposes and for the fabrication of various types of dental appliances, of forming digital images of teeth, or of entire sections of the mouth of a patient. In current practice, digitization is generally performed by scanning actual teeth or by scanning cast models of teeth formed using standard molding and casting techniques.
A dental impression is a negative replica of a given area of the oral cavity. The area replicated may be composed of either hard or soft tissues or both. Dental impressions are typically formed within the mouth of a patient, using an elastic material or a thermo-plastic material, such as a reversible hydrocolloid, that softens above a certain temperature. Other materials used may include various silicone or polysulfide-, and polyether-based synthetic rubber materials. Examples of dental impressions to which the instant invention may advantageously be applied are shown in FIG. 1. Dental impressions are useful for diagnostic purposes and may be used to cast molds, the casted molds being essential in the fabrication of various types of dental appliances, as will be further discussed in the disclosure that follows. In order to cast an accurate copy of teeth or parts of the mouth, plaster or dental stone is typically placed within an impression mold after the mold has been impacted by the patient.
The existing procedures for dental restoration involving the use of dental scanners typically proceeds as follows: First, a tray including an impression mold is inserted into a patient's mouth to create an impression of the patient's teeth and gingival tissue of the upper and lower jaw. The tray is held stationary until the impression material hardens capturing a negative, or reverse image of the teeth and gingival tissue. The tray is then removed from the patient's mouth, retaining the three dimensional impression of the teeth and gingival tissue of both the upper and lower jaw as well as their relative positions.
Following the creation of the impression, two separate plaster (gypsum) models must be produced by poring plaster into the impression mold. Pouring plaster into the impression mold allows separate positive models (top and bottom) of the oral structures to be created. These positive models made from the negative impression mold are referred to as “casts,” and, for dental scanners, are generally made from gypsum. The two separate models are held together in a mechanical jig when it is necessary to consider the relative alignment of the upper and lower dental features, as is often the case in dentistry and orthodontia. This process requires skill and is time-consuming.
Coordinate measuring machines (CMMs) are employed to determine the coordinates, in some specified frame of reference, of points on the surface of a workpiece. CMMs may be employed, for example, for digitizing or imaging that may be useful in the process of replicating a prototype for various manufacturing applications. The salient parts of a CMM include a stage, or a series of stages, for moving the object to be characterized, a probe for measuring the distance to a point on the surface of the work piece relative to a fiducial position, a control or computing system, and measurement software for converting the measurements into a meaningful format for the intended application.
One limitation imposed by existing CMMs is that even the most versatile optical sensors are unable to digitize on vertical or very steep angles measured with respect to the optical axis (or ‘line of sight’) of the probe. ‘Vertical’, in this case, refers to the surface of the scanned body lying parallel to the optical axis of the probe. An ‘undercut’ refers to a negative angle relative to the line of sight of the probe. Some applications, however, such as dental surface profiling for purposes of reconstruction, orthodontics, etc., as well as digitization of plastic parts, molds, etc., require measurements on vertical walls or low angle undercuts. Dental impressions, in particular, entail blind holes and sharp angles that are notoriously difficult, if not impossible, to digitize using standard CMM techniques.
As used herein, a body characterized as ‘complex’ is one having vertical walls or low angle undercuts. The use of prior art technology to scan a complex body requires orthogonal scanning of the object about multiple (typically 5) axis. As used herein, ‘orthogonal scanning’ refers to scanning of the line of sight of a probe entirely within a single plane normal to an axis of rotation. This method, while algorithmically simple, requires very large travel on the scanning stages making the equipment very expensive.
Another prior art solution to the problem of small (or zero, or negative) angles with respect to the probe line of sight entails performing non-orthogonal scanning by using a 2-axis angular arm. In this case the whole sensor is rotated, and both the complex arm and the requisite large travel ranges add to the cost of such systems. Yet another prior art solution for scanning complex bodies requires changing the sample position to allow direct line of sight for each feature. In this case very complex reconstruction software is required to merge the individual scans by ‘best fit’ of complex surfaces. The results of the ‘stitch’ depend on the quality of the data, the size and shape of common features used for references, and the robustness of the algorithms. Typically, operator intervention is required, both during the scanning (otherwise 2 or more motorized axis are required on the sample fixture) and during data processing. In some cases involving dental applications or plastic parts with smooth surface features, it is very difficult to find the right fit and reference items to register successive ‘views’ of the object. One solution requires ‘gluing’ registration features (‘balls’) to the sample.
Dental impressions have not been amenable to digitization for a number of reasons, primary among those reasons being that concave surfaces (and, indeed, abrupt vertical shafts, in some circumstances) are complex, in the sense defined above, prohibiting triangulation scanning processes. Moreover, the translucent optical properties of polymers typically used as impression materials do not provide clear reflecting surfaces and give rise to blurring. Thus, an automated and robust solution to the problem of digitizing complex bodies is acutely desirable as applied to the digitization of impressions, particularly dental impressions.